Systems and methods used to quantify the attributes or performance of an object play a crucial role in manufacturing the object, in designing products and processes that use the object, and in describing the object to consumers.
Over the years, various lighting industries have developed a number of systems or methods for quantifying the color and intensity of a light source. Such systems and methods rely on metrics (systems of measure based on a particular standard) and measurements (numerical values representing an amount, extent, or size determined by measuring) that are established by regulatory agencies, standards-producing bodies, industry stakeholders and individual organizations. The Metropolitan Gas Act of 1860, for instance, quantified the intensity of a burning candle to a known standard, introducing the standard definition of the metric “candlepower”. In 1931, the International Commission on Illumination introduced the CIE 1931 XYZ Color Space and XYZ color coordinates. The CIE 1931 color space created a metric for describing the perceived color of an object based on a set of mathematical coordinates. The CIE 1931 color metric is based on three visual response functions (a function is a relation between two sets in which one element of the second set is assigned to each element of the first set, as in the expression y=2x) describing the relation between color and intensity for the three types of cone cells in the human eye. These are known as the color matching functions and result in a color representation comprised of three values (a value is a particular magnitude, number, or amount) known as tristimulus values. From the CIE tristimulus values metrics like color correlated temperature (CCT), color rendering index (CRI), CIE (x,y), lumen, dominant wavelength and MacAdam ellipse may be measured or derived. These metrics, which quantify the appearance of lighting systems to human observers under specified conditions, have been used by manufacturers, designers and customers to grade products, calculate the performance of the products in new applications, and compare products from competing sources, enable manufacturers, designers and customers to grade products, calculate the performance of the products in new applications, and compare products from competing sources.
The aforementioned and widely used luminous metrics are well suited for quantifying the color and intensity of an object under specific illumination and observing conditions by a human observer. A problem arises using these metrics for manufacturing SSLs and designing lighting systems based on SSLs because there are many applications and processes where the SSL is not directly observed by the human eye. The present invention overcomes this problem of misapplication of metrics.
Furthermore, implicit assumptions in these metrics about the illuminant, field of view, ambient light, pupil dilation, and the relevance and accuracy of the Color Matching Function (CMF) contribute errors when these metrics are used for many light sources, particularly LEDs, HBLEDs and the other SSL sources. The dominant wavelength and luminous intensity metrics assume a human observer in daylight is observing a light source through a restricted 2 or 10 degree field of view. These conditions are often not accurately reproduced during testing and are rarely appropriate to the manner in which light sources are actually viewed when assembled into a final product. These metrics suffer from a phenomenon known as metamerism which is the inability of a human observer to discern a certain mixture of different colored light sources from each other. All of these issues contribute uncertainty to the measurement of spectral properties of SSLs adversely affecting precision and repeatability of measurements. The present invention overcomes these limitations.
These problems have less impact for lights producing a continuous spectrum (a classical black body emitter) such as the tungsten filament found in a traditional light bulb. However, traditional light color and intensity metrics have proven inadequate to quantify the color and intensity of SSL sources for design, manufacturing and assembly processes. For example, SSL sources such as HBLEDs are used as the primary source of light emission. Unlike a tungsten filament, HBLEDs are not emitters of black body radiation. An LED radiates light by band-gap radiative recombination of electrons and holes in a compound semiconductor. The spectral characteristics of the emitted light from a SSL are significantly different from a black body radiation source. Characterizing the color and intensity of a SSL light source is fundamentally incorrect using the traditional metrics because the underlying physics are fundamentally different. The present invention overcomes the problem.
A typical manufacturing process for a SSL (hereinafter using LED as an example) begins with the manufacturing of a LED on a wafer substrate. These substrates are inspected for physical and optical defects, and the SPD of LED emissions are recorded at various points on the wafer and converted to metrics that are used to determine the uniformity and optical characteristics of the wafer or die. Data collected during this evaluation is commonly used in two ways. First, to control product quality, the data is compared against quality standards to determine how well the wafer and its die meet quality standards. The quality of the wafer (determined by the number and nature of the defects and the optical output) determines if the wafer is allowed to continue in the manufacturing process and determines the ultimate usability of the wafer. The second use of the data is for manufacturing process improvements. The data collected during this evaluation is correlated to specific process inputs. Once the correlation is determined, these process inputs can be controlled and manipulated to improve process yield and reduce non-uniformities. The uncertainty of traditional light metrics used for SSLs and the unsuitability for use of the same in subsequent manufacturing process steps increase the range of variation of manufacturing processes. The present invention reduces this range of variation thereby leading to improvements in manufacturing processes of SSLs and related lighting system design and manufacture.